1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a laser beam printer.
2. Related Background Art
In general, an image forming apparatus such as a copier or a printer has image formation means and fixing means. The image formation means is for directly forming or transferring an unfixed toner image corresponding to image information on a recording material by a suitable image forming principle or process such as an electrophotography process or an electrostatic recording process. The fixing means fixes the toner image formed and bore on the recording material.
In general, a heat-fixing device is used as fixing means, which fixes an unfixed toner image on a recording material by heating the image. Thermal roller type fixing devices are employed most often.
A thermal roller type fixing device is equipped with a fixing roller, which serves as a heat-fixing rotor, and an elastic pressure roller, which has an elastic layer and which serves as a pressure rotor. The fixing roller and the elastic pressure roller rotate while pressed against each other. A recording material carrying an unfixed toner image is introduced into a press-fit nip portion between the fixing roller and the pressure roller, so that the image is fixed by heat from the fixing roller as the recording material is held and transported between the two rollers.
The fixing roller is heated by a heater serving as heating means, and a temperature of the fixing roller is adjusted to reach a given fixing temperature (temperature during printing) by temperature adjusting means. While an image forming apparatus is on standby, the temperature adjusting means controls the fixing roller temperature to a given standby temperature, which is set lower than the given fixing temperature, enabling the image forming apparatus to start printing immediately after receiving a command to print. Some known image forming apparatuses are capable of changing the fixing temperature (temperature during printing) and standby temperature of their fixing rollers in accordance with an environmental temperature (see Japanese Patent Application Laid-open No. 06-278308, for example).
If the elastic pressure roller of the above-described fixing device is also heated by the heater serving as heating means and is adjusted to have a given temperature, a toner image can be fixed well on rough-surface paper or thick paper, which would otherwise be difficult to print on.
However, a pressure roller that has a thick elastic layer loses the elasticity of the elastic layer at an accelerated rate through heating. Therefore, heating the pressure roller with a priority on improved fixing performance cuts short the service life of the pressure roller whereas giving priority to extended service life of the pressure roller lowers the performance of fixing an image on rough-surface paper and thick paper.
In addition, the pressure roller is pressurized at a given pressurizing force as well as the fixing roller and, if the pressure roller is heated while a certain load is applied, the elasticity of the elastic layer is degraded at an accelerated rate in a portion where the load is applied.